1. Field of the Invention
This invention relates to range binoculars, more particularly, the invention relates to a pair of binoculars having an optical system that measures distance and an optical system for observation.
2. Background Art
Typically, conventional range binoculars emit an infrared ray as a distance-measuring laser beam, collimate and aim the emitted infrared ray at a target, then receive a reflected ray from the target, measure the flight time taken from the ray-emitting point to the reflected ray-receiving point, and determine the distance from the observation place, i.e., the ray-emitting point to the target.
Conventional range binoculars are provided with a beam splitter, which permits an infrared ray to be transmitted, but does not permit visible rays to be transmitted between the objective lens of the optical system for observation and the erecting prism. As such, the infrared ray beam goes to the beam splitter, is reflected, and returns to the objective lens. The beam is collimated by the objective lens and transmitted to the target. Furthermore, in the conventional range binoculars, visible rays entering the objective lens for observation go through the beam splitter and advance to the eyepieces at which an image is formed.
The beam splitter used in the conventional range binoculars is required to assure the reflection of the distance-measuring infrared ray thereon and, at the same time, the transmission of the observed visible rays through itself at a high transmittance. Thus, the beam splitter is required to have a high performance.
To produce such high performance beam splitters is technically difficult, thereby resulting in high production costs. Consequently, conventional range binoculars using such beam splitters are rather expensive.
Furthermore, such high performance beam splitters are not able to transmit all of the visible rays reaching the beam splitters. Thus, conventional range binoculars, due to the presence of the beam splitter, inevitably reduce the transmittance of the observed visible rays and the resolving power thereof. As a result, images are degraded using conventional range binoculars.
The focusing mechanism of binoculars is of an independent feeding type (IF Type) or a central feeding type (CF Type). It is well known in the industry that focusing is easier with the CF Type binoculars than with the IF Type binoculars. In CF Type binoculars, an internal focusing lens is inserted between the objective lens and the erecting prism. The user focuses by moving the focusing lens. Therefore, the CF Type binoculars must be provided with the internal focusing lens and a means for moving in a space between the objective lens and the erecting prism.
However, it is very difficult and almost impossible to find a space to place an internal focusing lens in such conventional range binoculars, because the beam splitter is already fixed between the objective lens and the erecting prism. Thus, conventional rangefinder binoculars can hardly use the CF Type focusing systems, which is disadvantageous.